Gilbert signed the 6-year guaranteed contract back in 2008 — but according to new court filings in his family law case, obtained by TMZ Sports, Arenas will get his last installment on October 31st (the contract was paid out over 8 years).

It was good while it lasted … and Gilbert knew it when he spoke to our photog about the contract in 2013.

It continues:

The problem, Arenas says … is now that he’s no longer getting NBA checks, he’ll only earn $169,895 per year in investment income … not enough to live the BALLER lifestyle he’s used to.

So, Arenas says he has to make some serious cuts — and one of the items on the chopping block is the private school where he sends his kids … which runs $91,600 per year.

According to the site, Arenas said, “I anticipate that the children will need to transfer to public schools in 2017 and beyond.”